Carrier current protective system



June 1947. H. WQLENSNER ET AL 2,422,570

CARRIER CURRENT PROTECTIVE SYSTEM Filed March 9, 1945 7 EFCq 34 ransm/ fer EEC Eeceiver ATTORNEY Patented June 17, 1947 UNITED STATES PATENT OFFICE CARRIER CURRENT PROTECTIVE SYSTEM Herbert W. Lensner, East Orange, and James F.

Chapman, Madison, N. J., assignors to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application March 9, 1945, Serial No. 581,874

Claims. 1

Our present invention relates to a carrier-cur rent or other pilot-channel phase-angle-detecting relaying system, adapted to protect a section of a three-phase transmission-line against faults. Our present invention is an improvement over the system shown in an application of Mehring, Goldsborough and Lensner, Serial No. 534,846, filed May 10, 1944, Patent No. 2,408,868, granted October 8, 1946.

One of the problems in connection with a carrier-current relaying system of the type just mentioned has been the problem of providing a positive-contact carrier-starting circuit which operates at a high speed and is not affected by any irregularity in the mechanical performance of the fault-detector, No feasible fault-detector element is known, which is capable of always making a firm or solid contact when it responds, particularly near the minimum pickup-value of the element. In a carrier-current relaying system which compares the phases of a current responsive quantity at the two ends of the protected line-section, particularly in a system which produces a succession of tripping impulses on alternate half-waves of the local line-current unless opposed by a restraining impulse received by carrier, at fluttering carrier-starting contact produces intermittent transmission of carrier, which has caused incorrect tripping as a result of the failure of the proper restraining impulses which should have been transmitted by carrier from the other end of the protected line-section.

There has also been a potential source of trouble due to the possibility of starting the succession of tripping impulses before starting the succession of carrier-transmitted restraining impulses, if the circuits were not nicely adjusted.

A further requirement is that the carrier-current transmitter must be susceptible of being started by any auxiliary function, such as communication or telemetering, except during the time of a fault which requires unhampered carrier-current control in order to correctly operate the protective relays.

Our present invention has, for its object, the provision of novel means for meeting these re quirements and overcoming the inherent objections of carrier-current protective relaying systems of the class described.

A more specifically stated object of our invention is to provide a fault-detecting back-contact for starting carrier-current transmission quickly and positively, without any fluttering of the contact, even at the balance-point of the relay, and to start the production of the succession of intermittent operating or tripping forces in response to a make-contact of the same fault-detector, thus obtaining the advantage of the time-difference between the breaking of a back-contact and the making of a make-contact of the fault-detector relay.

.Other objects of our invention will be apparent from the detailed description and claims, and from the accompanying drawing, wherein the single figure is a diagrammatic view of circuits and apparatus embodying our invention in a preferred form of embodiment.

In the drawing, we show the terminal equipment for only one terminal of a three-phase transmission line H, which is connected to a bus l5 through a three-phase circuit-breaker [8. Only one terminal equipment is illustrated, because the equipments at the other line-terminal or terminals are, or may be, identical with the illustrated equipment. The circuit-breaker I6 is illustrated as having a trip-coil TC, and an auxiliary make-contact breaker-switch Ilia.

The three-phase line-current is derived by means of a bank of line-current transformers H, which respond to current-flow in the protected line-section, at the terminal in question. This three-phase line-current is'fed into any suitable phase-sequence network or filter l8, preferably a network which derives a single-phase controlvoltage in response to a composite function of more than one phase-sequence component of the line-current at the relaying terminal, for responding to a plurality of different kinds of faults on the transmission line. Such a network l8 may advantageously respond to the positive-sequence plus zero-sequence line-current component, as shown in the Harder Patent 2,183,646, granted December 19, 1939, and assigned to the Westinghouse Electric & Manufacturing Company.

The output terminals 23 and 24 of the network l8 are utilized to energize the operating coil of a fault-detector FD, which is intended to be representative of any multi-responsive fault-detector means, or any equivalent combination of fault-detector means, adapted to be responsive to a plurality of different kinds and phases of groundand phase-faults on the three-phase transmission system. This fault-detector FD is utilized to detect the presence of any one of a plurality of different kinds of faults, preferably all different kinds and phases of faults, whether such faults occur within the confines of the protected-line-section, or outside of said protected line-section.

The control is shown by schematic, or across the line diagram, between positive and negative bus-terminals marked and Each relay or electrically operated switching-device is indicated by a separate letter-designation or legend, which is applied to the operating-coil and to all of the contacts of the relay, as a convention for symbolically tying the various relay-parts together. Arrows are used to indicate how the various parts of each relay are connected together. All relays and switches are shown in the deenergized position,

The current-responsive sequence-network I8 is utilized to produce a succession of substantially fiat-topped restraining voltage-impulses of substantially constant magnitude during the positive half-cycles of the single-phase control-voltage which is produced in the network-terminals 23 and 24, and also to produce a succession of substantially fiat-topped operating voltage-impulses of substantially constant magnitude in response to the negative half-cycles of the controlvoltage. To this end, we preferably utilize the same means which is shown in the aforesaid Mehring et al. application.

Thus, as shown in the drawing, we provide two a gas triodes or other grid-controlled gas tubes V1 and V2 of a sustained-discharge type; that is, of a type in which the grid fires the tube, or starts the discharge, but is unable to extinguish the tube or interrupt the discharge. The grids of these tubes V1 and V2 are connected to the respective output-terminals 23 and 24 of the network l3. An intermediate voltage of the outputterminals of the network 18 is derived from two serially connected resistors RI and R2, which are connected across the network-terminals 23 and 24. The connecting point 25 between these resistors is connected to a negative battery-terminal or bust through a C-battery E0. The C- battery E is so connected as to make the point more negative than the negative battery-terminal or, in general, so as to make the point 25 have a potential too negative, by a predetermined amount, to cause the tubes V1 and V2 to fire, under the impressed anode-cathode voltageconditions.

The cathode-circuits 28 and 2? of the gas tubes V1 and V2 are connected to the negative batteryterminal through cathode-resistors R3 and R4, respectively. The anode-circuit 29 of the first gas tube V 1 is connected to the positive battery-terminal through a plate-resistor R5. The anode-circuit 32 of the second gas tube V2 is connected to the positive battery-terminal through a plate-resistor R6, conductor 3! and a make-contact 32 of the fault-detector FD. The two anode-circuits 29 and 38 of the gas tubes V1 and V2 are joined by an interconnecting circuit containing a capacitor CI.

The two gas tubes V1 and V2 are thus connected in a so-called trigger circuit which operates as follows. During control-voltage half-cycles of one polarity, which we may call the positive halfcycles, the filter-terminal 2-3 is positive. This filter-terminal 23 is also the grid-terminal of the first gas tube V1. At an early stage in these positive half-cycles, the positive voltage of the network-terminal 23, with respect to the intermediate point 25, becomes more positive than the blocking bias of the C-battery E0, or at least sumciently positive to cause the first gas tube V1 to fire. It will be understood that the gas tubes V1 and V2 have such characteristics that, when they are once fired, or when current is once started in their plate-cathode circuits, such platecathode current will continue to flow until the voltage applied across the plate and cathode terminals of the tube is reduced to zero or reversed, even for a moment.

At the beginning at the next half-cycle of the output-voltage of the network l3, which we may call a negative half-cycle, the other networkterminal 24 becomes positive with respect to the intermediate point 25, and fires the second gas tube V2,

Before the firing of the second tube V2, the potential of its plate-circuit 30 was substantially the potential of the positive battery-terminal assuming that the fault-detector contact 32 is closed. On the other hand, the potential of the plate-circuit 29 of the first, tube V1 was at a somewhat more negative value, due to the voltage-drop in the plate-resistor R5 of the first tube. When the second tube V2 fires, the potential of its plate-circuit 30 tends to drop to the same potential as the plate-circuit 29 of the first tube, but the voltage-charge on the interconnecting capacitor Cl causes the anode-circuit 29 of the first tube V1 to momentarily drop to a value which is more negative than the potential of the cathode-cir cuit 26 of said first tube V1, thus extinguishing the first tube V1 in the moment required for the discharge of the interconnecting capacitor Cl. In the next half-cycle, the first tube V1 fires again, and in turn extinguishes the second tube V2 by momentarily causing a negative voltage to exist across its plate-cathode terminals.

The function of the interconnecting capacitor Ci, which shunts the previously firing gas tube when the second tube begins to fire, is preferably supplemented by two capacitors C3 and C4, which are connected in shunt across the respective cathode-resistors R3 and R4 of the two gas tubes V1 and V2. The effect of these shunting-capacitors C3 and C4 is to short-circuit the associated cathode-resistor, R3 or R4, at the first instant of firing of the associated gas-tube, V1 or V2, as the case may be, thus momentarily bringing the anodepotential of the newly fired tube to a value which is more negative than the steady-state anodepotential of the tube which was previously firing. The interconnecting capacitor Cl, previous to the firing of the newly fired tube, was charged in such polarity as to momentarily tend to hold the anode-potential of the previously firin tube more negative than the anodapotential of the newly fired tube.

The combined effects of the three capacitors Cl, C3 and C4 is to strongly depress the anodepotential of the tube which was firing, at the first instant of firing of the other tube, making the anode-potential of the first tube momentarily more negative than its cathode-potential, thus extinguishing the tube. At the same time, the shunting-capacitor C3 or 04, as the case may be, of the tube that is being extinguished, momentarily holds up its cathode-potential to a value close to the value which it had when the tube was firing, thus assisting in maintaining the reversed tube-voltage for the instant necessary to extinguish the tube.

As explained in the aforesaid Mehring et al. application, the voltage-drops across the two cathode-resistors R3 and R4 are utilized to produce two different effects. The voltage-drop across the cathode-resistor R3 of the first gas tube V1 is utilized to produce half-cycle impulses of square-topped positive voltages for supplying a plate-voltage which is sufiicient for initiating and maintaining the operation of an oscillatortube OSC of a carrier-current transmitter 33, by connecting the plate-circuit 34 of the oscillatortube OSC, through a radio-frequency choke RFC-J, to a conductor 35, which is connected to the cathode-circuit 26 of the first gas tube V1. The cathode of the oscillator-tube 050 is connected, at 36, to the negative battery-terminal The voltage-drop across the cathode-resistor R4 of the second gas tube V2 is utilized to apply an operating voltage-component from the cathode-circuit 21 of the second tube V2 to the gridcircuit 31 of a relay-tube V4, which is shown near the bottom of the drawing and which will be subsequently described. A voltage-drop resistor R1 is included in the connection between the oathode-circuit 21 of the second trigger-tube V2 and the grid-circuit 31 of the relay-tube V4,

The carrier-current transmitter 33 is connected to one of the phase-conductors C of the protected line-section l4 through a coupling-transformer 38 and a coupling-capacitor 39.

The carrier-current equipment also includes a receiver 40 which is coupled to the coupling capacitor 39 through a coupling transformer M. The receiver 40 includes a detector-tube or re-- ceiver-tube REC, having a plateor anode-circuit 42 which is connected to the positive batteryterminal through a radio-frequency choke RFC2, and an alarm-device 43. The receivertube REC also has a cathode-circuit 44 which is connected at 45 to the negative battery-terminal through a tap 46 on a potentiometer 41.

The plate or anode-circuit 42 of the receivertube REC is coupled, by means of a capacitor C5, to a point 48 which is connected to the cathodecircuit 21 of the second tube V2 through a large, capacitor-charging resistor CCR. The point 48 is also connected, through a capacitor C6, to a conductor 49 which is connected to the cathodeterminal 50 of the left-hand diode of a doublewave rectifier-valve V3. The plate-circuit of this left-hand diode is connected to the grid-terminal 31 of the relay-tube V4, and to the voltage-drop or load resistor R1. The other terminal of the load-resistor R1 is connected to the cathode-circuit conductor 21 of the second gas triode V2, previously described. The right-hand diodecircuit 5! of the double-wave rectifier-valve V3 is connected, in the reverse polarity, between the circuits 21 and 49.

The load-resistor R1 is shunted by a radiofrequency by-pass capacitor BPC.

The relay-tube V4 is provided with a cathode circuit 52 which is connected to an intermediate point of a potentiometer 53 which is energized across the battery-terminals and The relay-tube V4 is also provided with a plate-circuit 54, which is connected to the positive batteryterminal through the primary winding of a relay-coupling transformer 55, the secondary of which is connected, through a rectifier-bridge 56, to the operating coil R of a tripping-relay R. The relay R is provided with a make-contact R, which is shown near the top of the drawing, in series with the trip-coil TC of the circuit-breaker The make-contact 32 of the fault-detector ED is also connected in the tripping circuit or the circuit-breaker It, said tripping circuit being traceable from the positive battery terminal through the fault-detector make-contact 32, the conductor 3|, and the tripping-relay make-contact R,-to the trip-coil TC, and thence through 6 the breaker-switch lBa to the negative batteryterminal In operation, the carrier-current energy, from both the local and distant transmitters, is received by the receiver-tube REC, so as to produce a plate-cathode current through this tube during periods when the carrier-current energy is being received.

When no carrier-current energy is being received, the anode-terminal 42 of the receivertube REC is practically at the potential of the positive battery-terminal (-1-), and hence the capacitor C5 is charged in accordance with the potential-difference between said anode-terminal 42 of the receiver, and the cathode-terminal conductor 21 of the second gas triode V2, as indicated by the signs and at the capacitor C5. This last-mentioned conductor 21 has a potential which is utilized as the operating-voltage for the grid-circuit 31 of the relay-tube V4, this operating-voltage being the voltage-drop through the cathode-resistor R4 of the second gas triode V2, whenever the latter is firing.

When the carrier-current energy is received, the receiver-tube REC becomes conducting, pulling down the potential of its anode-terminal 42 to a point which is more or less close to the potential of the negative battery-terminal thus more or less short-circuiting the capacitor C5, and causing it to discharge, drawing current through the load-resistor R1 and the left-hand diode of the rectifier-valve V3, said diode being connected in such polarity as to permit currentilow in the direction from the conductor 21 through the resistor R1 to the conductor 31, and thence through the left-hand diode to the conductor 53 and the capacitors CB and C5. At the same time, a much smaller current flows through the much larger capacitor-charging resistance CCR, which is utilized to charge the capacitor C5.

During the periods when no carrier-current energy is being received, in the illustrated form of embodiment of our invention, the receiver plate-circuit 42 again becomes quite positive, so that the right-hand diode-circuit 5| of the rectifier-valve V3 becomes conducting and charges the capacitor C6, as indicated by the signs and at the capacitor C6, thus causing this capacitor C6 to act as a voltage-doubler for doubling the effective voltage of the capacitor C5.

When, therefore, carrier-current energy is again received on the next half-cycle of the linefrequency current, the two capacitors C6 and C5 discharge through the load-resistor R1, thus producing a negative or restraining voltage-drop in said load-resistor R1, making the conductor 31, and hence the grid of the relay-tube V4, negative with respect to the potential of the cathodecircuit conductor 21 of the second tube V2. The reception of carrier-current thus causes the capacitors C6 and C5 to discharge, producing 2, voltage-drop in the load-resistor R1, making the grid of the relay-tube V4 more negative, and thus effectually preventing this tube from operating in response to the operating-voltage which is produced by th current-flow in the cathode-resistor R4 of the second gas tube V2.

The radio-frequency or carrier-frequency component of the plate-voltage of the receiver-tube REC is lay-passed from the load-resistor R1 by the by-passing capacitor BPC.

The receiver-tube REC preferably has a constant-current characteristic, so that whenever its grid permits plate-current to flow, its plate-current will have an approximately constant value. Thus, the half cycles of receiver plate-current, during which carrier-current energy is being received by the receiver-tube REC from the distant carrier, transmitted from some other line-terminal, are of an approximately fixed magnitude, regardless of carrier-current attenuation. Hence the restraining voltage-impulses in the resistor R1 are of an approximately fixed magnitude. The receiver plate-current impulses which are received from the distant carrier are of approximately the same magnitude as th half-cycle impulses of plate-current which are produced when carrier-current energy is being received from the local transmitter, even though the local signals may be the stronger.

It is preferable, also, that the relay-tube Vt shall have a constant-current characteristic, so that its plate-current will be constant, Without sensitive dependence upon the precise magnitude of its grid-voltage. Thus, the exact amount of the restraining voltage, produced in the load-resistor R? by the receipt of carrier-current energy, is not important, so long as said restraining voltage is greater than the operating voltage, or the voltage-drop in the resistor R4, by a safe margin.

t is further to be noted that the only carriercurrent response of ay moment is the response to the distant carrier, that is, the carrier-current impulses which are transmitted from some other line-terminal or terminals. The carrier-current energy received from the local carrier-current transmitter is immaterial, because, by the very nature of the control, it is always transmitted (and received) during the half-cycles alternating between the half-cycles when the operating impulses of the second gas triode V2 are produced.

The grid-voltage of the relay-tube V4 is thus made up of three components. First, there is a negative grid-bias consisting of the voltage between the potentiometer-tap 52 and the negative battery-terminal, which is suflicient to bias the grid of the relay-tube V4 so that no plate-current flows in said tube when ther is no restraining or operating voltage present. A second component of the grid-voltage of the relay-tube V4 is the operating voltage, in the form of positive voltage-impulses produced whenever the cathode-circuit current or" the second gas tube V2 flows through the cathode-resistor R4. The third gridvoltage component of the relay-tube V4 is the restraining voltage, produced by the discharge of the capacitors C5 and C5 through the resistor R1 whenever carrier-current energy is being received, although the restraining impulses which are received from a distant line-terminal are the only ones of importance.

Since the relay-tube V4 will be operated, or carry a plate-current, only when its grid is sufficiently positive with respect to its cathode, a plate-current will flow in the relay-tube V4 only during the positive half-cycles of the grid-voltage of said tube, that is, only when the local operating-impulses of the second-valve cathode-circuit conductor 2'! and its cathode-resistor R4 are not opposed by the restraining impulses received from a distant line-terminal.

When there is an internal fault, accompanied by fault-currents which are in phase with each other at the several line-terminals, the platecurrent of the relay-tube V4 takes the form of a succession of square-topped half-cycles corresponding in timing to the line-frequency halfcycles when the second gas tube V2 is firing, thus energizing th local tripping-relay- R and cansing a local tripping-operation.

In the case of an external fault, with linecurrents exactly out of phase with each other, the grid-biasing voltage of the relay-tube V4 is entirely negative, and the plate-current of the relay-tube V4 is zero, meaning no response of the relay R, and hence no tripping-operation.

In accordance with our present invention, the first gas tube V1 is normally prevented from operation by having its plate-circuit 29 connected, through a conductor 51 and an inductance L, to a back-contact 58 of the fault-detector FD, and thence to the negative battery-terminal The second gas tube V2 is normally prevented from operation by the previously described makecontact 32 of the fault-detector FD.

We also utilize the make-contact 32 of the fault-detector FD, and the conductor 3| which is connected thereto, to energize an auxiliary circuit 59 which contains the operating coil CS of a contactor-switch CS. This contactor-switch CS has a back-contact CS which is utilized in an auxiliary carrier-starting circuit 60, which can be traced from the positive bus through a keying device K, said CS back-contact, and a dropping resistor R8, to the conductor 35, which is connected to the transmitter-oscillator OSC through the radio-frequency choke RFC-l The CS back-contact opens in response to the makecontact 32 of the fault-detector FD, so as to prevent any unwanted transmission of carrier (un-v der the control of the keying device K), while there is a fault on the system which requires the use of the carrier for obtaining a correct relaying operation,

Our use of the back-contact 58 of the faultdetector FD to connect the plate-circuit 29 of the first gas valve V1 to the negative bus provides a quick and positive control of said first gas valve V1, because the FD back-contact opens positively Whenever the fault-detector FD responds, and it is not subject to the same degree of chattering as the FD make-contact 32, particularly during faults which are barely severe enough to cause a response of the fault-detector FD. This fault-detector back-contact 58 normally holds the plate-circuit 29 of the first gas tube V1 at the potential of the negative bus so that it is immaterial Whether the cathodecircuit 25 of said tube has the same potential as the negative bus Or a somewhat more positive potential due to the keyer-controlled carrier-starting circuit 60 being energized and causing a voltage-drop through the cathode-resistor R3 of said first gas tube V1.

The firing of the first gas tube V1 starts carrier-current transmission by applying the voltage-drop of its cathode-resistor R3 to the transmitter-oscillator OSC through the circuit-connection 35. Thus, carrier is started by the opening of the back-contact 58 of the fault-detector FD, and since the receipt of carrier produces the restraining voltage on the relay-tube V4, this restraining voltage is thus promptly established and positively maintained, without interruption by chattering of the fault-detector contact. Also, there is a certain slight time-delay between the opening of the fault-detector back-contact 58 and the closing of the fault-detector makeecontact 32, because it takes a certain small amount of time for the moving element of the fault, detector to move far enough to close the contact 32. In our improved system, this make-contact 32 is utilized to initiate the firing of the second 9; gas tube V2, which produces the voltagedrop in its cathode-resistor R4, which is utilized as the operating voltage for the relay-tube V4, This improves the coordination of the relay on faults external to the protected line-section.

If the carrier equipment is located some distance from the fault-detector FD, the controlcircuit leads of the back-contacts 58 of the faultdetector FD will hav an appreciable capacitance C between them. This capacitance across the carrier-starting contacts 58 may be sufiicient to prevent the operation of the trigger-circuit of the gas-valve V1. The reactor L is included in the circuit 51 in order to isolate the capacitance C" of these fault-detector leads from the first gas valve V1. This reactor presents sufiicient impedance to the square-topped output of the triggercircuit to prevent the capacitance C from adversely afiecting its operation.

The inductance L and the capacitance C form a resonant circuit which will cause an oscillation every time the first gas tube V1 fires. In order to damp this oscillation, we shunt the inductance L with a resistance R. This oscillation causes a varying voltage to be applied to the plate-circuit 29 of the carrier-starting gas tube V1, and if the oscillation is of sufiicient magnitude, it can actually reverse the polarity of the voltage which is applied to this tube, which would stop the firing of the tube and would stop the transmission of carrier. The frequency of such oscillation is expressed by the following formula:

, lin ii "2r LC 4L In this equation, if

ii LC iL the circuit will be critically damped and will not oscillate. The resistor R across the inductance L is chosen to make the circuit critically damped for the largest value of lead-capacitance C which is anticipated in actual installations.

While we have described our invention in a preferred form of embodiment, we wish it to be understood that many changes of omission, addition and modification may be made by those skilled in the art, without departing from the essential principles of our invention. We desire, therefore, that the appended claims shall be accorded the broadest construction consistent with their language and the prior art.

We claim as our invention:

1. Terminal equipment for a pilot-channel phase-angle-detecting relaying-system adapted to protect an alternating-current transmissionline against faults, comprising fault-detector means having make-contacts and back-contacts, said fault-detector means being responsive to line-conditions at said terminal, means operative to develop a single-phase control-voltage havin a phase which is responsive to the line-current at said terminal, pilot-channel means operative to transmit a succession of restraining impulses and to make them effective at another line-terminal or terminals in response to positive half-cycles of said control-voltage, local control-means operative to develop a succession of operating impulses in response to negative half-cycles of said control-voltage, means for making said pilotchannel means inefiectual in response to a closed position of back-contacts of said fault-detector means, means for making said local controlmeans ineffectual in response to an open position or make-contacts of said fault-detector means, and phase-angle-detecting relay-means operative to respond to said operating impulses when they are not effectively opposed by restraining impulses received from a distant line-terminal.

2. Terminal equipment for a pilot-channel phase-an-gle-detecting relaying-system adapted to protect an alternating-current transmissionline against faults, comprising fault-detector means having make-contacts and back-contacts, said fault-detector means being responsive to lineconditions at said terminal, means operative to develop a single-phase control-voltage having a phase which is responsive to the line-current at said terminal, two gas tubes of the sustaineddischarge type, each tube having trigger-acting control-circuit means for firing the tube, directcurrent plate-cathode-circuit energization-means for said two gas tubes, interconnecting impulsingmeans between the plate-cathod circuits of said two gas tubes for responding to the moment of firing of either tube in such manner as to so impulse the effective plate-cathode voltag across the other tube as to extinguish said other tube, means for so applying said control-voltage to the control-circuit means of the two gas tubes that the tubes fire during half-cycles of opposite polarity of said control-voltage, pilot-channel means operative to transmit a succession of restraining impulses and to make them effective at another line-terminal or terminals in response to the intermittent operation of a first one of said two gas tubes, local control-means operative to develop a succession of operating impulses in response to the intermittent operation of the second gas tube, means for preventing the application of a firing-starting plate-cathode-circuit voltage to said first gas tube in response to a closed position of back-contacts of said fault-detector means, means for preventing the application of a firingstarting plate-cathode-circuit voltage to said second gas tube in response to an open position of make-contacts of said fault-detector means, and phase-angle-detecting relay-means operative to respond to said operating impulses when they are not effectively opposed by restraining impulses received from a distant line-terminal,

3. Terminal equipment for a pilot-channel phase-angle-detecting relaying-system adapted to protect an alternating-current transmissionline against faults, comprising fault-detector means having make-contacts and back-contacts, said fault-detector means being responsive to line-conditions at said terminal, means operative to develop a single-phase control-voltage having a phase which is responsive to the line-current at said terminal, two gas tubes of the sustaineddischarge type, each tube having trigger-acting control-circuit means for firing the tube, direct-current plate-cathode-circuit energization means for said two gas tubes, interconnecting impulsing-means between the plate-cathode circuits of said two gas tubes for responding to the moment of firing of either tube in such manner as to so impulse the effective plate-cathode voltage across the other tube as to extinguish said other tube, means for so applying said control-voltage to the control-circuit means of the two gas tubes that the tubes fire during halfcycles of opposite polarity of said control-voltage, the plate of a first one of said gas tubes being normally connected to the positive terminal of the energization-means through a resistance and being normally connected to a negative potential through back-contacts of said fault-detector means, the plate-cathode-circuit of the second gas tube including make-contacts of said fault-detector means, pilot-channel means operative to transmit a succession of restraining impulses and to make them effective at another line-terminal or terminals in response to the intermittent operation of said first as tube, local control-means operative to develop a succession of operatin impulses in response to the intermittent operation of said second gas tube, and phase-angle-detecting relay-means operative to respond to said operating impulses when they are not effectively opposed by restraining impulses received from a distant line-terminal.

4. The invention as defined in claim 2, characterized by the control-circuit leads of the backcontacts of the fault-detector having an appreciable capacitance, and a resistance-shunted inductance in series-circuit relation between said back-contacts and said first gas tube.

HERBERT W. LENSNER. JAMES F. CHAPMAN.

REFERENCES CITED 7 The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,144,494 Harder Jan. 17, 1939 2,408,868 Mehring et a1 Oct. 8, 1946 2,406,617 Lensner Aug. 27, 1946 Bostwick et al Aug. 27, 1946 

